In today's vehicles and particularly with the increasing popularity of hybrid and electric vehicles and stop-start systems, the increasing electrical load presents a challenge to the battery. Many vehicle breakdowns caused by the electrical system can generally be avoided by knowing the precise state of the battery. The battery must be able to provide enough energy to crank the engine and be available as a passive power source to support the vehicle's auxiliary electrical systems.
One known battery monitoring device which can communicate with a vehicle's power management system using a local interconnect network (LIN) is the Freescale MM 912J637 module. This known monitoring device is described in Freescale document number MM 912_637D1 Rev. 3.0 1/2012. This device employs a two-channel analog to digital converter for simultaneous measurement of battery voltage and current and an independent analog to digital converter for temperature measurement. Current measurement is achieved with the aid of a low ohmic shunt resistor connected between the negative pole of the battery and the chassis ground of the vehicle. The device is directly connected to and supplied from the battery and located directly on the battery's negative pole. Battery current is measured by measuring the voltage drop across the shunt resistor and is defined as the differential voltage between two inputs of the current sensing channel of the device. This current sensing channel employs sigma delta modulation techniques in order to realise a high resolution analogue to digital conversion process.
While this type of battery monitoring device performs satisfactorily for low voltage (for example lead-acid) batteries, it does not perform so well with high voltage batteries (for example lithium-ion used in electric and hybrid vehicles). There is a requirement for speed of current measurement in the case of high voltage batteries which conventional battery current measurement devices cannot meet. Speed requirement is driven by the number of battery cells to be monitored individually. For lead-acid batteries, a single measurement of the cell stack is sufficient. For Li Ion, between 6 and 12 cells must be measured within a given time window. There are further requirements in the case of high voltage battery monitoring concerning the relatively wide input signal range and the need for accuracy of measurement.